56 kbps modem systems are quickly becoming the preferred choice among consumers (for client-end modems) and internet service providers (for server-end modems). 56 kbps modem systems employ pulse code modulation (PCM) technology to facilitate higher downstream transmission data rates to an end user. For example, FIG. 1 depicts a conceptual diagram of a typical 56 kbps communication path using current PCM modem technology. A central site, such as an internet service provider (ISP) 100, is digitally connected to a telephone network 130 through a transmitter 110 and a receiver 120 resident at an ISP modem 105. The network 130 is connected to a local loop 150 through a central office line card 140. The line card typically has a PCM codec implemented therein. The local loop 150 is connected to the user's personal computer (PC) 170 at the user's site through the user's modem 160. As can be appreciated by those skilled in the art, the connection between the ISP modem transmitter 110 to the telephone network 130 is a digital connection that supports a typical data rate of about 64 kbps. Since the parameters of the telephone network 130 and line card 140 are dictated and set by the operating specifications of the network (and particularly the use of the .mu.-law or A-law signal point constellations), the central site transmitter 110 is configured to transmit the digital data in a particular way to fully exploit its digital connection to the network.
Transmission power limitations for telecommunication systems (including modem systems) may be mandated by regulatory bodies such as the Federal Communications Commission (FCC). For example, current FCC regulations on modem transmissions over the public telephone network in the United States require that average power levels do not exceed -12 dBm0. Accordingly, the particular codewords associated with each transmission session, and the manner in which such codewords are transmitted, may be selected to ensure that a specific transmit power level is not exceeded. On the other hand, unnecessarily low power levels may cause a low system signal to noise ratio (SNR), which can result in an increased probability of errors and otherwise poor system performance.
In general, modem or other data communication systems that are not limited to the transmission of specific signal points may address the transmit power limitations in a relatively straightforward manner. For example, a modem system that is not limited to a particular set of transmit signal points may simply scale its output to comply with any regulatory restrictions. In contrast, in signal point limited systems (that may utilize signal point constellations designated by a receiver), no such scaling is possible and the constellation itself dictates the total average transmit power. Accordingly, the constellations designed by the receiver determine whether the transmitter complies with the transmit power regulations.
As mentioned above, regulatory bodies may place limits on the total average power utilized for a given data communication session. Because the transmit power limitations may vary from country to country, the digital modem in a 56 kbps system may initially provide the maximum transmit power limit to the analog modem such that the analog modem can design an appropriate signal point constellation set. Accordingly, after the appropriate signal point constellations are selected, the total average transmit power may be computed by the analog modem to ensure that the transmit power of the constellation set does not exceed the maximum transmit power limit. However, without an independent verification of the transmit power associated with the signal point constellations, the digital modem may utilize a signal point constellation set that, due to computational errors on the part of the analog modem, exceeds the maximum transmit power limit.
As mentioned above, conventional 56 kbps modem systems perform constellation design and power calculation at the analog modem (i.e., the client-end modem) after obtaining a maximum transmit power limit from the digital modem (i.e., the server-end modem). Unfortunately, the manner in which the analog and digital modems calculate the total average transmit power may vary from one device to the next. In other words, the same transmit power formula may not be rigidly followed by all modern devices. Consequently, the analog and digital modems may generate inconsistent transmit power calculations for the same signal point constellations.
Even if the analog and digital modems are in agreement with respect to the transmit power formula, practical operating limitations (such as processor bit resolution or the use of finite precision arithmetic) may introduce round off errors to the power verification procedure. Thus, like the above situation where two different transmit power formulas are employed, the analog and digital modems may obtain different transmit power results for the same signal point constellation set. The calculation of different results utilizing the same transmit power formula may adversely affect any verification routine later performed by the digital modem. For example, the digital modem may reject signal point constellations for exceeding the transmit power limit even though the analog modem designed the constellations to be within the transmit power limit and even though the analog modem may have already performed an initial verification.
Present 56 kbps modem systems may not consider transmission power levels during training procedures. For example, training sequences may be designated in advance without regard to any transmit power limitations imposed on the digital modem. Furthermore, present systems may not effectively select the training signal points in accordance with current operating conditions such as the presence of robbed bit signaling or digital pads. Such digital impairments may have a negative affect on the quality of the training procedure, especially if the training signal points are influenced by the digital impairments.